Electrical – Testing circuit breakers

If there's no current to the fixture or switch, then the fault is upstream. The fact that it was flickering indicates a failing connection, possibly arcing somewhere, which can be a fire hazard. You should check all the junctions on this circuit from the switch back to the breaker until you locate one with a good hot, neutral, and ground connection. And then somewhere between that junction and the next failing junction is your issue. It may be a bad connection that can be repaired at the end, or it may be a bad line and you'll need to rerun that segment of the circuit.

Over the internet, we have no way to be more specific. It entirely depends on how the home is wired and where the fault is located, which we don't have enough information to determine.

It seems to me, after reading all the stuff in your first question and on here that the problem is inside your cloths dryer. There is a good chance that some part of the heater coil is becoming shorted out and causing huge amounts of current to flow when the coil becomes hot.

The probable reason that it is the main breaker that trips is due to that breaker being the most sensitive to the sudden current surge short that occurs in the dryer. It may be an indication that that main breaker needs replacement but before even going down that road it is time to unplug the dryer and do a through investigation of its innards, particularly its heater unit and the wiring thereof.